Optical fiber transmission systems are subject to distortion related to loss, noise, and nonlinearities in both the fiber and the modulation and amplification devices. One of the more deleterious forms of nonlinear distortion is that due to chromatic dispersion. Chromatic dispersion in optical fiber is typically characterized by a linear (non-flat) group delay parameter. The group refractive index of the fiber at optical frequencies near a given optical carrier frequency varies approximately linearly with wavelength or optical frequency about the carrier. This finite linear group delay imposes a quadratic phase rotation across the signal frequency band.
Approaches currently used to reduce the effects of chromatic dispersion include: (1) reversing the effects of chromatic dispersion in the optical domain, (2) reversing the effects in the electrical domain after optical detection and (3) reducing the transmission bandwidth of the optical signal on the fiber. The first is based on purely optical methods where the effects of group velocity dispersion are reversed while the signal is still in the optical domain. Adding dispersion compensating fiber in the transmission path is one common approach. Other optical methods include compensation by differential time delay of the upper and lower sidebands of the modulated signal, see A. Djupsjobacka, O. Sahlen, "Dispersion compensation by differential time delay," IEEE Journal of Lightwave Technology, vol. 12, no. 10, pp. 1849-1853, October 1994; spectrally inverting the signal at the midpoint of the transmission path, see R. M. Jopson, A. H. Gnauck, R. M. Derosier, "10 Gb/s 360-km transmission over normal-dispersion fiber using mid-system spectral inversion," Proceedings OFC'93, paper PD3, 1993; and pre-chirping the transmitted signal in an external modulator, see F. Koyama, K. Iga, "Frequency chirping in external modulators," IEEE Journal of Lightwave Technology, vol. 6, no. 1, pp. 87-03, January 1988 and A. H. Gnauck, S. K. Korotky, J. J. Veselka, J. Nagel, C. T. Kemmerer, W. J. Minford, D. T. Moser, "Dispersion penalty reduction using an optical modulator with adjustable chirp," IEEE Photonics Technology Letters, vol. 3, no. 10, pp. 916-918, October 1991.
The second approach, in which dispersion effects are reversed in the electrical domain, is based on coherent transmission and heterodyne detection followed by equalization in the electrical domain. Homodyne detection is only effective on single sideband signals. If homodyne detection were attempted with a DSB signal, the upper and lower sidebands would overlap upon detection and the phase information would be lost and the higher modulation frequencies severely attenuated or distorted through partial or complete cancellation of various sideband frequencies. Some techniques used or proposed for post-detection equalization include microstrip lines, see K. Iwashita, N. Takachio, "Chromatic dispersion compensation in coherent optical communications," Journal of Lightwave Technology, vol. 8, no. 3, pp. 367-375, March 1990; microwave waveguides, see J. H. Winters, "Equalization in coherent lightwave systems using microwave waveguides," Journal of Lightwave Technology, vol. 7, no. 5, pp. 813-815, May 1989, and fractionally spaced equalizers, see J. H. Winters, "Equalization in coherent lightwave systems using a fractionally spaced equalizer," Journal of Lightwave Technology, vol. 8, no. 10, pp. 1487-1491, October 1990.
The third approach is to modify the transmission format where the baseband signal spectrum is compressed. These types of approaches, which reduce the transmission bandwidth required on the fiber to transmit a given bit rate, are generally implemented by modifying the line code format in order to reduce the effective bandwidth required to transmit or receive the data, see K. Yonenaga, S. Kuwano, S. Norimatsu, N. Shibata, "Optical duobinary transmission system with no receiver sensitivity degradation," Electronic Letters, vol. 31, no. 4, pp. 302-304, February 1995, and G. May, A. Solheim, J. Conradi, "Extended 10 Gb/s fiber transmission distance at 1538 nm using a duobinary receiver," IEEE Photonics Technology Letters, vol. 6, no. 5, pp. 648-650, May 1994.
More recently it has been shown that optical single sideband transmission (OSSB) can combat some of the deleterious effect of chromatic dispersion. OSSB provides a dispersion benefit directly by reducing the signal bandwidth and also by augmenting post-detection dispersion compensation. The generation, transmission and detection of single sideband (SSB) signals has been used for both baseband and the RF and microwave regions of the electromagnetic spectrum to reduce the bandwidth of the signal by a factor of two, by sending either the upper or the lower sideband. Generation and transmission of OSSB optical signals using a Mach-Zehnder modulator is shown in M. Izutsu, S. Shikama, T. Sueta, "Integrated optical SSB modulator/frequency shifter," IEEE Journal of Quantum Electronics, vol. QE-17, no. 11, pp. 2225-2227, November 1981 and R. Olshansky, "Single sideband optical modulator for lightwave systems," U.S. Pat. No. 5,301,058, 1994. Methods based on AM compatible radio modulators were outlined in Jan Conradi, Bob Davies, Mike Sieben, David Dodds and Sheldon Walklin, "Optical Single Sideband (OSSB) Transmission for Dispersion Avoidance and Electrical Dispersion Compensation in Microwave Subcarrier and Baseband Digital Systems", OFC 97 Postdeadline, February 1997, and M. Sieben, J. Conradi, D. Dodds, B. Davies, and S. Walklin "10 Gbit/s optical single sideband system" Electronics Letters Vol. 33, No. 11, pp. 971-973. These structures overcame the need for large added carrier in the transmitted optical signal by using approximations to time domain minimum phase signals with single sideband properties. This allowed the transmitted information to be directly modulated onto the optical electric field envelope while a special phase function was incorporated into the AM signal to cancel half of the transmitted information spectrum.
While in baseband SSB optical modulation a dispersion benefit accrues directly due to the fact that the transmitted signal spectrum has been reduced by a factor of two, the more significant advantage of optical SSB transmission is that the fiber dispersion can be compensated in the electrical domain after detection. This advantage is similar to that for heterodyne detection of DSB signals, but with SSB transmission and detection, the signal can be homodyned directly to baseband using carrier signal added either at the source or at the receiver and thus it can be directly detected with the phase or delay information of the transmitted signal preserved. This was shown in K. Yonenaga, N. Takachio, "A Fiber chromatic dispersion compensation technique with an optical SSB transmission in optical homodyne detection systems," IEEE Photonics Technology Letters, vol. 5, no. 8, pp. 949-951, August 1993, where integrated optical structures were used to generate single sideband tones for narrowband applications. In K. Yonenaga, No. Takachio, "Dispersion compensation for homodyne detection systems using a 10 Gb/s optical PSK-VSB signal," IEEE Photonics Technology Letters, vol. 7, no. 8, pp. 929-931, August 1995, a single sideband optical modulator was described for the purpose of transmitting two or more optical signals with different optical carrier frequencies on a single fiber. The purpose of transmitting the signals in a single sideband format is to permit these optical carrier frequencies to be spaced as closely as possible to the maximum modulation frequency. A fundamental disadvantage of this type of dispersion compensation is found in the fact that the carrier power added to the transmitted signal must be significant thus reducing the potential signal to noise ratio at the transmitter.